The present context is that of a communications network in which the nodes are organized as a ring, enabling them to communicate with each other. To be more precise, the nodes of the communications network are connected by two unidirectional rings, one for each rotation direction. This topology is particularly suitable for metropolitan area networks (MAN) because it is fault-resistant: in the event of a fault breaking both rings at the same point, a new logical ring can be formed by joining together the two physical rings upstream and downstream of the break.
The following disclosure relates to a mechanism for controlling access to one of the two rings or to the logical ring formed after a fault. A ring comprises one or more ring resources. In a time-division ring, a reference period is divided into time intervals of fixed duration, a time interval being used to transmit one or more packets. As used below the expression “transmission resource” refers to a subset of one of the resources of the ring accessible during a given time interval, also known as a transmission window. The ring is used to route various data streams, some of which have time constraints, for example conversation and audiovisual streams.
Numerous mechanisms have been proposed for controlling access to the transmission resource. To be effective, they must enable spatial re-use, i.e. the successive use of the same transmission resource for communications involving different parts of the ring. To achieve this, the transmission resource must be released by the destination node of the data transmitted during the transmission window as soon as the data is received.
Moreover, nodes must be prevented from monopolizing the transmission resource, for example because of a better position on the ring. With the nodes numbered from 1 to N in the rotation direction of the ring, if node 1 transmits a large volume of data to node N, a node situated between these two nodes may be refused access to the transmission resource, for example. In the absence of any control, node 1 can thus access the resource easily and potentially monopolize it. It is therefore necessary to regulate access of the nodes to the transmission resources. This access control must be fast to prevent streams with time constraints having to wait.
The Resilient Packet Ring mechanism defined by IEEE standard 802.17 consists in not blocking access to the transmission resource providing the sizes of the respective queues of packets waiting to be transmitted from the nodes are below a predetermined threshold. Such a mechanism is described in the article “IEEE 802.17 Resilient Packet Ring Tutorial” by F. Davik et al. published in IEEE Communications Magazine, March 2004. If a given node detects congestion, it reports it to the upstream nodes on the ring, i.e. the nodes that might insert traffic into the ring. Reception of this congestion information by one of the nodes of the ring causes it to reduce its own consumption of transmission resources. The system is thus reactive, i.e. action is taken in response to detecting congestion.
Choosing the predetermined threshold or congestion-detection threshold is not easy. Too low a threshold prevents full use of resources, some nodes reducing their own consumption of transmission resources when there is no real congestion. In contrast, too high a threshold increases the waiting times for some nodes, a node reacting only when the congestion-detection threshold is reached. Because of the random nature of traffic, there is no optimum threshold.
The prior art technique therefore fails to respond in a satisfactory manner to the problem of access to transmission resources in a time-division ring. With a threshold sufficiently high to enable effective use of the bandwidth, the mechanism can cause localized famines for some nodes, which may disrupt applications sensitive to waiting time, such as voice or video applications.
Patent application US2007/029744 proposes a mechanism for use by a node of a time-division ring in order to reserve a transmission window. In one implementation, the node reserves a transmission window as soon as its queue contains packets waiting for transmission. Alternatively, the node reserves the transmission window only if it detects congestion. The implementation described is unsatisfactory in that, once again, a node is able to monopolize a portion of the resources of the ring by reserving transmission resources in a quasi-continuous manner. It has the further drawback of using the reservation mechanism only when congestion is detected. The problem of defining the thresholds described above arises again.